1. Field of the Invention
The disclosures discussed herein relate to an electrochromic display device.
2. Description of the Related Art
Electronic paper has been increasingly developed as electronic media that replace ordinary paper. One of the features of the electronic paper may be that a display device itself is handled as ordinary paper, which differs largely from related art display devices such as cathode ray tubes (CRTs) or liquid crystal display (LCD) devices. Thus, the electronic paper may need to have characteristics largely differing from those of the related art display devices. For example, the electronic paper may need to be a reflective display device capable of exhibiting high white reflectivity and a high contrast ratio, displaying high definition images, exhibiting stable display memory effects, being driven by a low-voltage, having thin and lightweight properties, and being produced at low cost. Among these characteristics, it may be particularly desirable that the electronic paper include the white reflectivity and the contrast ratio as high as those of ordinary paper. Various types of display devices have been proposed as those suitable for the electronic paper. Examples of such display devices include a reflective liquid crystal display device, an electrophoretic display device, and a toner migration display device. However, any of the above examples may seem to have difficulty in displaying multicolor images while maintaining certain levels of white reflectivity and contrast ratio at the same time.
As a promising technology to also implement the reflective display devices, there is a technology utilizing “electrochromism”. Electrochromism is a phenomenon displayed by some materials of reversibly changing the color due to reversible redox induced when a voltage is applied. An electrochromic display device is defined as a display device utilizing color development and color reduction of the electrochromic compounds that induce such “electrochromism”. As described above, the electrochromic display device is a reflective display device, has display memory effects, and is capable of being driven by a low voltage. Accordingly, the electrochromic display device has been extensively studied as one of the promising candidates for the display device technology for use in electronic paper, from material development to device design.
Japanese Patent Application Publication No. 2010-033016 (hereinafter referred to as “Patent Document 1”), for example, discloses a multicolor display device, in which two or more display electrodes are arranged such that the display electrodes are separate from one another between a display substrate and counter pixel electrodes, and the display electrodes are provided with corresponding electrochromic layers. With this configuration, the display device, which is formed by simply layering the display electrodes and the electrochromic layers on the display substrate, is capable of implementing layered multiple color display of Y (Yellow), M (Magenta) and C (Cyan).
In the multicolor display device disclosed in Patent Document 1, the two or more display electrodes are separately arranged from one another between the display substrate and the counter pixel electrodes, and the display electrodes are provided with corresponding electrochromic layers. With this configuration, the electrochromic layers of the selected display electrodes are individually driven to develop or reduce corresponding colors. Accordingly, the electric resistance between the display electrodes arranged separately from one another may need to be higher than the electric resistance within each of the display electrode surfaces. Note that the “developing or reducing the color of the electrochromic layer” herein indicates the color development or color reduction of the electrochromic compounds that induce the “electrochromism” as described above.
If the electric resistance between the display electrodes is small, the electric current may flow into unselected display electrodes. That is, it may not be possible to drive the selected display electrodes to independently develop or reduce the color of the electrochromic layer of the selected display electrode. In Patent Document 1, there is proposed that an electrolyte permeable insulator layer be formed between the electrochromic layer and the display electrode in order to acquire insulation between the electrodes, and also the permeability of the electrolyte layer.
Further, in this configuration disclosed in Patent Document 1, after charging of the electrochromic layers to develop corresponding colors, the developed colors of the pixels are retained by forming an open-circuit configuration. However, the open-circuit configuration may not provide sufficient stability in the developed color of each of the electrochromic layers. That is, the developed colors of the electrochromic layers may be mixed with time. This may be because the electric charges of the electrochromic layers attenuate with time, and as a result, the balance in the electric charges between the electrochromic layers may not be retained. The degradation of the balance in the electric charges between the electrochromic layers may be particularly noticeable when the utilized electrochromic layers have a low charge retainability.
Further, in the electrochromic display device disclosed in Patent Document 1, it may be difficult to acquire sufficient insulation between the display electrodes while driving the selected electrochromic layers corresponding to the finely arranged counter pixel electrodes to independently develop or reduce the respective colors of the selected electrochromic layers (active matrix driving). That is, the developed color of the preceding electrochromic layer may have an adverse effect on the color developing operation of the succeeding electrochromic layer.